Such circuits serve for suppression of interference caused when two transmitters broadcast on adjacent frequency channels, and one of the transmitters overmodulates its transmission, so that its signal oversteps the boundaries of its channel. This degrades the reproduction quality of the signal received from the other, "stepped-on" transmitter. Atmospheric conditions and reflections from moving objects may also contribute to adjacent-channel interference.
It is conventional, especially in AM receivers, to regulate or vary the Intermediate Frequency (IF) bandwidth as a function of the signal strength received from the adjacent channel transmitter. When the adjacent, potentially interfering transmitter is strong, one narrows the bandwidth of the circuit tuned to receive the desired transmitter, in order to avoid picking up any of the interfering adjacent-channel signal. To do this, one provides two receiving circuits, each tuned to one of the two adjacent channels. Each receiving circuit feeds a respective rectifier. The rectifier output voltages are then used in the IF stage of the receiver to adjust the IF bandwidth tuning or filtering. The bandwidth regulation is thus accomplished by damping or coupling or specifying in the IF resonant or tuned circuit.